Denise, I don't know if this will help you, but it's worth a shot. Now that I'm on meds that give me a boost in mood I have finally been able to rationally look at my childhood. I too have hated myself most of my life and I hear a voice in my head that says, "I hate you; I'm going to kill you."
A couple of weeks ago I figured out why I hated myself. First of all I have never wanted to be a victim and abhor that mentality. As a child I was abused in several ways. I blamed myself for everything bad and good that happened to me. Why? It was a warped kind of coping. If I blamed myself, then I created an illusion of control over my out of control environment. I had it within my power, or so I thought, to make choices that would give me good and bad outcomes. So I blamed and took credit for everything. Actually I gave most of the good credit to God and just blamed myself for all the abuse. How I came to hate myself? For all the bad things that happened to me that were my fault and I should have "chosen" better to get a good outcome. It was a vicious cycle.
I still hear the hate you voice, but now I know it comes from that child trying to cope in a world where adults hurt her instead of protecting her. Now I get to be the adult and can care for myself.
You don't have to blame yourself any more. That child that you were and the adult that you are deserves to be nurtured. It wasn't your fault. You are a good person. I can tell from all your posts.
Maybe this has nothing to do with how you came to hate yourself, but there are more reasons that you might discover as you go through therapy or even read self-help books.
When you start to feel better on a good meds combo, I think you may be able to face these feelings of self-hate head on and come out the other side with a new understanding and an inner love for yourself...the innocent part of you. The child who was still full of wonder before the confusion of bad things happening set in.
Sending you big

.
Today I feel strange. I woke up at 5:30 and I usually get up at 10. I think I might be a bit hypomanic. Got the lowest scores ever on the depression and mania tests on this site: a 2 for each. Either I'm recovered from my last episode or I'm heading up.

I hope everyone has a good day.